Mark B Snyder

Performance of Rigid Pavements Containing Recycled Concrete Aggregates

State highway agencies in Connecticut, Kansas, Minnesota, Wisconsin, and Wyoming have successfully designed and constructed rigid pavements containing recycled concrete aggregate (RCA). Success has been attributed in part to the minimization of old mortar content in the RCA during recycling processes, thereby controlling the total mortar content of the new portland cement concrete (PCC) mixture, or to the achievement of higher-than-expected compressive strengths through adjustments in mix proportions, or both. There was no clear correlation between mortar content and cracking distresses in field investigations, although one project did exhibit significantly more slab cracking in the recycled pavement than in the corresponding control pavement. The increased cracking may have been due to the large differences in total mortar content between the recycled and control sections. In general, the recycled PCC pavements considered in this study have performed comparably with their conventional PCC pavement counterparts, including the recycled pavements that incorporated RCA derived from concrete affected by D-cracking and alkali-silica reactivity (ASR). There is, however, evidence of small amounts of localized recurrent ASR in the recycled Wyoming pavement. Whether this reactivity will eventually develop into widespread distress remains to be seen.

VALIDATION OF FLEXIBLE PAVEMENT STRUCTURAL RESPONSE MODELS WITH DATA FROM THE MINNESOTA ROAD RESEARCH PROJECT

A dynamic load test study was performed on instrumented asphalt and concrete pavement test sections at the Minnesota Road Research facility. Test variables included different types of vehicles (featuring various axle groupings, load levels, and tire pressures) operating at various speeds over different structural sections. Four flexible pavement sections were selected for inclusion, and the primary structural response measured was horizontal strain at the bottom of the asphalt layer. The test data suggest that the structural response of the four sections to varying loads was linear when other test parameters were held constant. The pavement sections also exhibited a pronounced viscoelastic behavior in response to changes in vehicle speed and load rate. This behavior was attributed mainly to the asphalt concrete layer. Changes in tire pressure did not significantly affect pavement behavior. The test data were used to validate a multilayer linear elastic pavement structural model. Sub-grade and base moduli inputs were back calculated from falling weight deflectometer data, and the asphalt modulus was determined experimentally from measured strain. This model adequately reproduced the strain distribution observed at the bottom of the asphalt layer under the passage of moving vehicles with certain limitations: the longitudinal strain asymmetry due to viscoelastic effects could not be reproduced, and the modulus of the asphalt layer had to be readjusted for temperature and vehicle speed. Also, it was not possible to calibrate or fit the model to observed longitudinal and transverse strains simultaneously because the pavement behaved more stiffly longitudinally than transversely.

REVIEW OF STUDIES CONCERNING EFFECTS OF UNBOUND CRUSHED CONCRETE BASES ON PCC PAVEMENT DRAINAGE

Recycled concrete aggregate (RCA) products are sometimes used as replacements for virgin aggregate products in concrete pavement structures. Recent concerns have centered on the deposit of RCA-associated fines and precipitate suspected of reducing the drainage capacity of RCA base layers and associated drainage systems. Environmental concerns have focused on the relatively high pH of the effluent produced by untreated RCA base layers. Several studies have examined these concerns and others; the results of some of these studies have not been published or publicized. The most relevant of these studies are summarized herein. These research efforts demonstrate that calcium-based compounds are present in most recycled concrete aggregates in quantities sufficient to be leached and precipitated in the presence of carbon dioxide. Precipitate potential appears to be related to the amount of freshly exposed cement paste surface. Thus, selective grading or blending with natural aggregates can reduce, but not eliminate, precipitate problems. It was also noted that insoluble, noncarbonate residue makes up a major portion of the materials found in and around pavement drainage systems. Washing the RCA products before using them in foundation layers appears to reduce the potential for accumulation of dust and other fines in the drainage system, but probably has little effect on precipitate potential. Field studies have shown that precipitate and insoluble materials can significantly reduce the permittivity of typical drainage fabrics but that attention to drainage design details can minimize the effects of these materials on pavement drainage.

Performance of Rigid Pavements Containing Recycled Concrete Aggregate: Update for 2006

A 1994 field survey was undertaken of pavements containing recycled concrete aggregate (RCA) constructed in Connecticut, Kansas, Minnesota, Wisconsin, and Wyoming. These pavements were resurveyed during the summer of 2006 to update their performance after they had been subjected to 12 more years of traffic. Additional pavements made with RCA from Illinois and Iowa were also observed in 2006. Although the recycled pavements contain higher mortar contents, there was no clear correlation between the higher total mortar content of RCA concrete pavements and cracking distresses in either survey, although one RCA concrete pavement did exhibit more cracking than the control pavement. Overall there was little difference between the 1994 and 2006 surveys. Several pavements were rehabilitated by the addition of dowels for load transfer. These pavements are performing exceptionally well and show that rehabilitation techniques normally applied to conventional concrete work effectively on recycled pavements. Laboratory evaluation of field cores showed that 10 of the 16 pavements surveyed exhibited evidence of alkali–silica reactivity. Eight of these pavements were shown to have significant remaining expansion potential and are expected to continue expanding. All pavements constructed with RCA from concrete showing alkali–silica reactivity and D-cracking exhibited field performance equivalent to their controls and pavements without distress. The recycled pavements have generally performed comparably with their controls. For instance, the present serviceability rating was found to be similar for the recycled and control sections.

Evaluation of Frost Resistance Tests for Carbonate Aggregates

D-cracking is a progressive distress associated primarily with the use of coarse aggregates that deteriorate when they are critically saturated and subjected to repeated cycles of freezing and thawing. The present study was undertaken to consider better acceptance criteria for concrete aggregates and to allow for the use of more local Minnesota aggregates through selected aggregate beneficiation techniques. Condition surveys of concrete highway pavements were performed to document the field freeze-thaw performance of selected aggregate sources representing a range of frost resistance. Cores were obtained from these sections for laboratory testing and evaluation, and coarse aggregates were obtained from the original sources for use in performing environmental simulation tests [i.e., variations of ASTM C666 and the Virginia Polytechnic Institute (VPI) single-cycle slow-freeze test] and correlative tests (i.e., absorption and bulk specific gravity, Portland Cement Association absorption and adsorption tests, Iowa pore index test, acid insoluble residue test, X-ray diffraction analysis, X-ray fluorescence analysis, thermogravimetric analysis, and the Washington hydraulic fracture test). The tests that provided the best correlation with field performance included a modification of ASTM C666 Procedure B (specimens prepared with salt-treated aggregates), the VPI single-cycle slow-freeze test, and the Washington hydraulic fracture test. Other test procedures were correlated with field performance to lesser extents. It was noted that petrographic examination of pavement cores can help to distinguish between D-cracking and other conditions that can produce distresses with similar appearances (e.g., distresses caused by secondary mineralization, embedded shale, poor mix design, and alkali-aggregate reaction).

Effectiveness of portland cement concrete curing compounds

Many different spray-on compounds are available for curing concrete, including newer products that are intended to address the environmental concerns associated with high volatile organic compound (VOC) contents. A laboratory study was conducted to examine the effectiveness of different types of curing compounds in retaining water for hydration, promoting concrete strength, and reducing permeability, relative to classic curing techniques such as plastic sheeting and ponding and relative to the use of no curing treatment. Comparisons of moisture loss, compressive strength, permeability, and capillary porosity were made for samples representing three high-VOC curing compounds, three low-VOC curing compounds, water curing, and plastic-sheet curing, and for samples with no curing treatment after 3 days and 28 days of curing. The performance of the six compounds tested varied greatly, but none of the compounds performed as well as the samples cured with water or plastic sheeting. All compounds performed better than samples with no curing treatment.

LIFE-CYCLE COST COMPARISON OF ASPHALT AND CONCRETE PAVEMENTS ON LOW-VOLUME ROADS: CASE STUDY COMPARISONS

The costs of pavement construction, maintenance, and rehabilitation are primary factors considered by most local agencies in the selection of pavement type [hot-mix asphalt concrete (HMAC) or portland cement concrete (PCC)] for new construction. The optimal use of agency funds for any given project can be determined only through an economic analysis of all associated agency costs and the performance of the pavement. Life-cycle cost analyses were performed on HMAC and PCC highway pavements in Olmsted and Waseca Counties, Minnesota. The Means Heavy Construction Historical Cost Index and the Minnesota Department of Transportation Surfacing Indices were used to convert all expenditures over time into equivalent constant-dollar values. Direct comparisons were made on roadway sections with similar traffic volumes, ages, and environmental conditions. For Olmsted County, the favored pavement type depended somewhat on the cost index values that were used in the analysis; however, index selection had no effect on the outcome for the Waseca County comparisons. When the results were normalized for traffic volumes (i.e., cost per lane mile per million vehicles carried), PCC pavements were clearly more cost-effective in all Olmsted County cases and all but one Waseca County case, regardless of the cost index value used. PCC pavements generally incurred significantly lower maintenance and rehabilitation costs than HMAC roadways in both counties.

Using Recycled Concrete as Aggregate in Concrete Pavements to Reduce Materials Cost

The main objective of this project was to evaluate the effects of using aggregate produced from crushed concrete pavement as a replacement for natural (virgin) coarse aggregate in pavement mixtures. A total of ten different concrete mixtures containing recycled concrete aggregate (RCA) were designed to meet the requirements of Indiana Department of Transportation (INDOT) specifications. These included three different RCA replacement levels (30%, 50% and 100% by weight of the natural coarse aggregate) and two different cementitious systems (plain system – Type I portland cement only and fly ash system – 80% of Type I portland cement and 20% of ASTM C 618 Class C fly ash). The scope of the project included the evaluation and comparison of several properties of RCA and natural aggregates, evaluation and analysis of the effects of RCA on concrete properties, and modification of aggregate gradations and mixture composition in an attempt to improve the properties of RCA concrete. All ten mixtures were first produced in the laboratory (trial batches) and were subsequently reproduced in the commercial ready-mixed concrete plant. Each mixture produced in the ready-mixed plant was used to prepare several types of specimens for laboratory testing. The tests performed on fresh concrete included determination of slump and entrained air content. The mechanical properties of the hardened concrete were assessed by conducting compressive strength, flexural strength, modulus of elasticity and Poisson’s ratio tests. Concrete durability was assessed using a wide array of measurements, including: rapid chloride permeability (RCP), rapid chloride migration (RCM), electrical impedance spectroscopy (EIS), surface resistivity, free shrinkage, water absorption test, freeze-thaw resistance and scaling resistance. The test results indicated that the properties of plain (no fly ash) concrete mixtures with 30% RCA as coarse aggregate were very comparable to (in some cases even better than) those of the control concrete (0% RCA). Although mixtures with 50% RCA showed a reduction in durability and mechanical properties of up to 36%, the test results still met INDOT’s specifications requirements. The mechanical properties of plain concretes made with 100% RCA were measurably lower (16%-25%) than those of the control concrete. It should be pointed out, however, that these properties were still above the minimums required by INDOT’s specifications except for one mixture in which the w/c was increased to 0.47 to achieve workability. The use of fly ash improved the strength and durability of RCA concrete, especially at later ages. In particular, the properties of concrete with 50% RCA coarse aggregate were similar to the properties of control concrete. Similarly, the mechanical and durability properties of the mixture with 100% RCA coarse aggregate and 20% fly ash were better than those of a similar mixture prepared without fly ash. Even though, when compared to the fly ash concrete with 100% virgin aggregate the mechanical and durability properties of the 100% RCA concrete were up to 19% and 35% lower, it still met minimum requirements imposed by INDOT’s specifications.

REFINEMENT AND VALIDATION OF THE HYDRAULIC FRACTURE TEST

The hydraulic fracture test was developed under the Strategic Highway Research Program to address the need for a more rapid, less expensive test for concrete aggregate freeze–thaw durability. Although the test concept appeared sound, the original test and analysis procedures were not sufficiently reliable and accurate to merit widespread adoption and implementation. Several follow-up research efforts have been performed, and each has resulted in improvements to the test. The results of the most recent study, which evaluated changes in both the test procedure (to include additional test sieves for better characterization of particle fractures) and the analysis procedures, are described. The “hydraulic fracture index” has been replaced by a model that predicts freeze–thaw test dilation as a function of the distribution of particle mass retained on the test sieves. This model was developed using data obtained from freeze–thaw and hydraulic fracture testing of 18 quarried carbonate and gravel aggregate sources; the resulting correlation is exceptional (r2 = 0.98). An additional improvement is the development of a large test chamber capable of handling aggregate samples five times larger than the original small chamber, which thereby allows aggregate durability characterization with a single test run. It is believed that the hydraulic fracture test is now ready for more broad-based validation testing and eventual widespread acceptance and implementation as an accurate screening tool for concrete aggregate freeze–thaw durability.

Using the Minnesota Accelerated Loading Facility to Test Retrofit Dowel Load Transfer Systems

The Minnesota Accelerated Loading Facility (Minne-ALF) is a laboratory-based, linear-loading, pavement test stand that simulates the passage of heavy loads moving at speeds up to 65 km/h over small, full-scale pavement test slabs. Moving wheel loads are simulated using a rocker beam controlled by hydraulic actuators. In this study, the passage of 40-kN single wheel loads was simulated at a rate of 172,800 loads per day. Demonstration tests were performed to (a) determine the effects of selected design and construction variables on retrofit dowel load transfer system performance, (b) measure the variability of Minne-ALF test results, (c) demonstrate the general usefulness of the Minne-ALF, and (d) identify the need for any additional test system modifications. Concrete slabs specimens were cast and dowels were installed in slots across cracks and formed joints. Test variables included joint face texture, repair backfill material, and dowel material and length. Test outputs included measurements of load transfer efficiency and differential deflection across the joint or crack. The effect of joint or crack face texture was great when the joint or crack remained tight. Load-carrying performance was improved by using Speed Crete 2028 in place of Minnesota Department of Transportation specification 3U18 (preproportioned bag mix) concrete backfill with similar joints and dowel bars. Load transfer was unaffected by the use of stainless steel–clad, fiber-reinforced plastic or grout-filled tubes in lieu of epoxy-coated structural steel dowel bars. Additional testing is recommended to examine the effects of dowel length and materials. This test program demonstrated the usefulness of the Minne-ALF; additional testing is planned.